34
2 Wind Tunnels and Other Aerodynamic Test Facilities
– The Damköhler number, Da =
K L
V ∞
, where K is the reaction rate of the species
considered, represents the ratio between the chemical reaction rate and the aerodynamic velocity. If this number is large, then over the distance L, a great quantity
of the considered species considered could form (in the algebraic sense), or in
more precise terms, the chemical reactions involving these species have had time
to advance. If the reaction rates are much greater than the aerodynamic velocities,
the chemical equilibrium has time to settle at any moment. On the other hand, if
this number is small, within the length L, the delay, or residence t time, is too
short for the chemical reactions to have time to advance appreciably. Then the
composition of the gas does not vary: it is said frozen. This number is of great
importance in the simulation of reactive hyperenthalpic flows (see Sect. 6.3).
2.5 Constraints of Testing in a Wind Tunnel
2.5.1 Effects of Blockage in the Test Section
The wind tunnel tests are performed in a test or working section, constituting of a
confined space, hence a risk of the influence of the walls on the flow around the
model: these are the wall effects which are very detrimental in transonic flows (see
Sect. 4.2). At subsonic speeds, the size of the test section defines the longitudinal
and transverse dimensions of the body to be studied. In general, the length of the test
section must be at least twice the total length of the model for a diffuser length at least
equal to the test section length. As far as transverse dimensions are concerned, the
test section blockage effect, defined by the ratio of the largest model cross-sections
area and that of the test section, should be less than 0.16. In any case, corrections to
the measurements may be necessary to take into account the influence of the localised
acceleration generated by the presence of the model or vehicle in the test section.
Wind tunnels for land vehicles are generally equipped with a suction systems aiming
at reducing the thickness of the boundary layer at the test section entrance, thus better
reproducing the actual flow conditions under and around the vehicle (see Sect. 3.3).
The purpose of wind tunnel tests is most often to reproduce the flight conditions
of an aircraft which operates in an unconfined atmosphere. However, in a guided test
section, solid walls constrain the flow, while in an open test section the air stream
is restricted to a jet developing in an external environment at constant pressure. If
the dimensions of the model are not small compared to those of the test section, or
the cross-section of the jet, the proximity of these solid walls or fluid boundaries
respectively modifies the flow around the model, which no longer corresponds to
that desired. When the size of the model is reasonably large (the limit depends
on the accuracy of the desired results), interference due to walls can be evaluated
and corrected to transpose to free flight conditions. For a given wind tunnel, the
advantage of such an approach is to allow the test of larger models (thus higher
Reynolds numbers), hence more detailed and accurate measurements.
2 Wind Tunnels and Other Aerodynamic Test Facilities
– The Damköhler number, Da =
K L
V ∞
, where K is the reaction rate of the species
considered, represents the ratio between the chemical reaction rate and the aerodynamic velocity. If this number is large, then over the distance L, a great quantity
of the considered species considered could form (in the algebraic sense), or in
more precise terms, the chemical reactions involving these species have had time
to advance. If the reaction rates are much greater than the aerodynamic velocities,
the chemical equilibrium has time to settle at any moment. On the other hand, if
this number is small, within the length L, the delay, or residence t time, is too
short for the chemical reactions to have time to advance appreciably. Then the
composition of the gas does not vary: it is said frozen. This number is of great
importance in the simulation of reactive hyperenthalpic flows (see Sect. 6.3).
2.5 Constraints of Testing in a Wind Tunnel
2.5.1 Effects of Blockage in the Test Section
The wind tunnel tests are performed in a test or working section, constituting of a
confined space, hence a risk of the influence of the walls on the flow around the
model: these are the wall effects which are very detrimental in transonic flows (see
Sect. 4.2). At subsonic speeds, the size of the test section defines the longitudinal
and transverse dimensions of the body to be studied. In general, the length of the test
section must be at least twice the total length of the model for a diffuser length at least
equal to the test section length. As far as transverse dimensions are concerned, the
test section blockage effect, defined by the ratio of the largest model cross-sections
area and that of the test section, should be less than 0.16. In any case, corrections to
the measurements may be necessary to take into account the influence of the localised
acceleration generated by the presence of the model or vehicle in the test section.
Wind tunnels for land vehicles are generally equipped with a suction systems aiming
at reducing the thickness of the boundary layer at the test section entrance, thus better
reproducing the actual flow conditions under and around the vehicle (see Sect. 3.3).
The purpose of wind tunnel tests is most often to reproduce the flight conditions
of an aircraft which operates in an unconfined atmosphere. However, in a guided test
section, solid walls constrain the flow, while in an open test section the air stream
is restricted to a jet developing in an external environment at constant pressure. If
the dimensions of the model are not small compared to those of the test section, or
the cross-section of the jet, the proximity of these solid walls or fluid boundaries
respectively modifies the flow around the model, which no longer corresponds to
that desired. When the size of the model is reasonably large (the limit depends
on the accuracy of the desired results), interference due to walls can be evaluated
and corrected to transpose to free flight conditions. For a given wind tunnel, the
advantage of such an approach is to allow the test of larger models (thus higher
Reynolds numbers), hence more detailed and accurate measurements.